Motorcycle Sprockets and Gearing, Part 1
The Basics
Gearing Basics
If you've ever ridden a bicycle with more than one gear (and I'm assuming nearly everyone reading this, at one point in time or another, has), you've experienced first-hand how sprocket size affects acceleration, road speed, and the amount of input force required to set the bike in motion. Indeed, the 'transmission' on most such bicycles is comprised of little more than a selection of closely spaced sprockets of differing tooth counts; shifting up or down simply moves the drive-chain from one sprocket to another. While our motorcycles don't operate in this particular manner, it does make the common bicycle an excellent tool for demonstrating, first hand, how sprocket selection affects motorcycle dynamics.
For example, consider what happens when you shift through the gears on your pedal-bike. Using the rear derailleur to switch sprockets on the rear wheel hub, you notice that moving to smaller sprockets makes pedaling more difficult. But you also notice that, so long as you maintain the same pedal cadence (speed), your bike goes faster. Likewise, as you go up in sprocket size, pedaling becomes much easier. But in order to maintain the same road speed, you're forced to pedal faster. On the front, the mechanics are nearly the same… only reversed. Switching to a larger front sprocket increases the amount of force required to turn the pedals, but also increases the bike's speed for a given pedal cadence. Switching to a smaller front sprocket requires less leg work, but you're forced to spin the pedals faster if you don't want to slow down.
In either case, the physics involved are identical. By changing the sprockets' sizes, we change the relationship between how many times the crank arm turns to how many times the rear wheel turns. As this relationship changes, so does the mechanical advantage offered by our gearing arrangement. To better understand mechanical advantage, think back to the last time you used a long breaker bar to loosen a rusted nut. The amount of force required to break the nut loose with the breaker bar was certainly much less than what you would have had to use if you had stuck with that 6-inch ratchet. But the distance the end of the breaker bar had to travel - the point where force was applied, was much greater. That's mechanical advantage in action, and it always involves a trade-off between the amount of force needed to achieve a certain affect, and the distance over which that force must be applied.
On our bicycle, as the front sprocket size is reduced in relation to the rear (or the rear sprocket size is increased in relation to the front), mechanical advantage is increased; less input force is required at the pedals, but the crank must travel through more revolutions to turn the rear wheel a given amount. Swapping out to a larger front sprocket (or smaller rear) reduces mechanical advantage, having the opposite effect; the input force required increases, but the rear wheel rotates further with each turn of the crank. On a chain driven motorcycle, these concepts are exactly the same… the only differences being the internal combustion engine replacing our legs, and the inability to dynamically change sprocket sizes while we ride (though the same affect is realized on a motorcycle through the transmission).
Low Gear, High Gear, Tall Gear, Short Gear
In common terms, as we increase mechanical advantage (smaller front sprocket / larger rear), we 'go down' in gearing. As we decrease mechanical advantage (smaller rear sprocket / larger front), we 'go up'. These terms may seem counterintuitive at first. But they make sense when we look at how gear ratios are typically expressed. For any given combination of sprockets, the resulting gear ratio is calculated by dividing the number of teeth on the front sprocket by the number of teeth on the rear. So, for purposes of example, if we combined a 15 tooth front sprocket with a 30 tooth rear, the overall gear ratio would be expressed as a 15/30, or .50. If we later decided to swap out the rear sprocket for a 60-tooth unit, we'd increase our mechanical advantage significantly, but our gear ratio - 15/60, or .25, would decrease. Going up in gearing, which can also be called going 'taller', means going to a higher calculated gear ratio. Going down in gearing, or going 'shorter', equates to a decreased gear ratio.
Another thing to note about this fractional method of expressing gear ratios is that it tells you how many times the rear wheel turns in relation to the input. On a bicycle, a 15/42 gear setup would require 15 full pedal revolutions to turn the rear wheel 42 times. On a motorcycle, it would require 15 revolutions of the transmission output shaft for every 42 turns of the rear wheel.
Acceleration
Most mountain bikes are capable of very low gear combinations, mainly of use on steep ascents where great mechanical advantage is needed to overcome gravity. On flat ground, however, these sprocket combinations are essentially worthless. The minimal pedal resistance allows our legs to spin up as fast as they can go in a very short period of time, but the bike barely moves. In such a low gear, we can accelerate very quickly, but our road speed at 'redline' (which, for the average recreational cyclist, is probably less than 120 revolutions per minute), is quite slow.
In contrast, bicycles developed for road racing generally incorporate very high gear ratios, allowing for incredibly fast road speeds. But pushing these big gears requires a lot of muscle, a lot of endurance, and - to reach top speed - a lot of time (relatively speaking). Once again, these concepts apply just the same to motorcycles as they do their human-powered counterparts.
In a nutshell, lowering gear ratios generally increases the rate of acceleration a machine is capable of, but limits its top speed. Higher gears extend theoretical top speed, but slow the rate at which speed is gained. The optimum balance between the two is determined by the specifics of your particular application.
Theoretical Top Speed vs. Actual Top Speed
I know I just got through saying the gearing determines top speed, but there are times when things aren't always so black and white. Bear with me. All will soon be made clear.
Let's once again consider our trusty bicycle. Assuming we're on a relatively flat surface, two factors limit just how fast we can go… how fast our legs can spin, and how hard we can pedal. For purposes of example, let's say our maximum pedal cadence is 120 rpm. No matter how easy the pedaling is, the fastest we can move our legs is 120 revolutions each minute (or 2 turns per second). With this 'redline' defined, we can now determine our theoretical top speed, which would be the speed we'd reach if we pedaled at 120 rpm in our highest gear. Assuming our big sprocket up front had 50 teeth, and our smallest sprocket in back had 10, our top-gear ratio would be 50/10, or 5.0. This ratio would equate to 5 turns of the rear wheel for every one turn of the pedal crank - which at 120 rpm - would result in 600 revolutions of the rear wheel each minute. Measuring the circumference, or distance around, the rear wheel, and then multiplying that figure by 600, would then give us the distance travelled each minute… our theoretical top speed.
But why is it 'theoretical'? Why not just call it 'top speed'? Because there's still one variable that hasn't been defined… and that's how hard we can pedal. To illustrate this point, the next time you're on your bicycle, see how fast you can pedal in your lowest gear, where turning the crank is nearly effortless. Once you've reached the highest cadence possible, start shifting up through the gears until you're on your largest sprocket in front, and your smallest sprocket in the rear. Are you still able to spin as fast as you could in low gear? Chances are pretty good the answer is no, and that your cadence has dropped off by a large margin. Why? Because between the decreased mechanical advantage of your high gearing, combined with exponential effect of wind resistance at speed… your 'engine' isn't strong enough to pedal that hard, that fast. If you had a stronger engine - theoretically - you could go faster. But you don't. The speed you managed to reach is your 'actual' top speed, and any gearing over and above that isn't going to help you go any faster. (And if you can spin as fast in high gear as you can in low… keep buying bigger front sprockets until you can't! I'm trying to make a point here!)
The difference between theoretical top speed and actual top speed is important when discussing motorcycle gearing, due to the fact that many production motorcycles, sportbikes in particular, have theoretical top speeds well in excess of their actual top speeds. As an example, recent models of the Suzuki GSX-R600, with stock gearing, are theoretically capable of speeds in excess of 180 mph at redline in 6th gear. But in practice, no bone-stock 600 has the power to reach such a speed, regardless of distance. Any gearing allowing speeds over and above what the bike can practically pull in a reasonable distance is wasted gearing. Even motorcycles that can reach their theoretical top speed with standard gearing often can't do so in a reasonable distance. If the longest straight on your favorite road or track is ¼ mile, how fast you're able to go by the 2-mile mark doesn't really matter. Not only that, gearing for a top-speed in excess of what you commonly use robs you of acceleration that could otherwise propel you to your practical top speed… faster.
In most applications, we recommend using a practical and useful top-speed as the starting point for sprocket selection. But there are some cases where gearing above the anticipated top speed makes sense. With some of the more powerful bikes, rear wheel spin and front wheel lofting become limiting factors that can result in slower lap times. If you're already having trouble keeping the front end on the tarmac, or are flirting with a highside on the exit of every turn, gearing down even further may not be to your advantage. In such situations, gearing up, possibly even in excess of the useful top speed, may give a rider enough control to actually go faster with less acceleration.
Likewise, commuters, pleasure riders, and even endurance racers often find higher gear ratios desirable due to decreased engine wear, reduced vibration, and improved fuel mileage.
The next installment of 'Motorcycle Sprockets and Gearing' is coming. Please check back soon.
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